(1) The IL-3-dependent myeloid progenitor cell line, 32D, was transfected with expression vector containing human cDNAs for epidermal growth factor (EGF), platelet-derived growth factor (PDGF) or colony-stimulating factor-1 (CSF-1) receptor tyrosine kinases. Exposure of the 32D transfectants to their respective ligands triggered proliferation coupled with monocytic differentiation. Expression of receptors lacking tyrosine kinase domains in 32D cells induced longterm proliferation without monocytic differentiation. These receptors included those for erythropoietin (Epo), IL-2 and GM-CSF. Our results suggest that the four hematopoietic receptors analyzed may utilize the same signal transduction pathway in 32D cells and that the four tyrosine kinase-containing receptors may evoke separate or additional pathways for signal transduction in this system. (2) We have demonstrated that phosphatidylinositol-3 kinase associates with tyrosine-phosphorylated cellular substrates and hematopoietin receptors after ligand-induced stimulation of a variety of receptors including those for IL-2. IL-3, IL-4, Epo and GM-CSF, suggesting that activation of this enzyme may play an integral role in mitogenesis of cells within the hematopoietic lineage. (3) We have generated a murine monoclonal antibody (MAb) directed against the human a PDGF receptor (PDGFR). This MAb recognizes the human a PDGFR by immunoprecipitation, immunoblot analysis and immunofluorescence on live cells expressing the alpha PDGFR. The MAb was shown to neutralize PDGF-induced biological responses. Conversely, PDGF blocks the ability of the MAb to recognize the receptor. These results suggest that the epitope recognized by this MAb is in the extracellular PDGF binding domain and may be useful in the diagnosis or treatment of diseases involving overexpression of PDGF or the alpha PDGFR.